Audio-frequency amplifier



June 1949- 'E. WATKINSON EI'AL 2,473,265

AUDIO-FREQfiENCY AMPLIFIER Filed Dec. 4, 1946 300 VOLT 500 \vOLT ANDREWILLEM .STORM CHARLES ORDON SMITH &

ERIC WATKINSQN INVENTOR6 Patented June 14, 1949 AUDIO-FREQUENCYAMPLIFIER Eric Watkinson, Hinders Park, South Australia, Australia,Andre Willem Storm, Eindhoven, Netherlands, and Charles Gordon Smith,Sydney, New South Wales, Australia, assignors to Hartford National Bank& Trust Company, Hartford, Conn., as trustee Application December 4,1946, Serial No. 714,074

In Australia August 28, 1945 Section 1, Public Law 690, August 8, 1946Patent expires August 28, 1965 6 Claims.

The present invention relates to audio frequency amplifiers.

It is well known that, with class B and class AB audio frequencyamplifiers, the current re- 2 nected to ground via the two resistors RIand R2, which form part of the resistive network including te resistorsRI, R2, R3 and R4. These resistors are each by-passed by a condenser ofquirements of the output tubes vary over a con- 5 suitable capacity.

siderable range. To enable an amplifier of this The voltage for theanodes of the 807 tubes type to function at maximum efiiciency, it hasis obtained from the positive termin l of th hitherto been necessary tooperate the amplifier 500 volt supply while the screen electrodes offrom a direct current source having very good the 807 tubes and theanode of the 6V6 tube are regulation and/or to make use of a stabilizing10 fed from the positive terminal of the 300 volt tube or tubes, thecurrent consumption of which varies inversely with respect to thecurrent consumption of the amplifier.

It is the primary object of the present invention to provide a class Bor class AB amplifier which requires a power supply having onlymoderately good regulation. This object may be accomplished in apreferred form of the invention by utilizing one or more of the tubes inthe initial stages of the amplifier as stabilizing tubes in addition totheir normal use as voltage amplifying or driving tubes.

The invention will now be described with the aid of the accompanyingdrawings which gives the circuit diagram of an amplifier comprising twotetrode output tubes, for example, of the 807 type driven by a drivertube, which may be any grid-controlled tube but which is preferably ofthe 6V6 type.

In this amplifier the plate and the screen grid of the 6V6 tube areconnected together so that it operates as a triode. The tube istransformer coupled to, and drives the grids of the 807 tubes whichoperate in a push pull circuit in a well known manner.

The voltages for the electrodes of the tubes are preferably obtainedfrom two power supplies, one of the order of 500 volts and the other ofthe order of 300 volts. These power supplies may be derived from thealternating current mains by means of rectifiers, which, by reason ofthe improvement resulting from the use of the in vention, do not requirea high degree of regu lation, nor does the current supplied thereby needto be filtered to any great extent.

The high voltage windings connected to the rectifiers may be wound onthe same core and may be energized by a single primary winding. A filterconsisting only of condensers or of condensers and series resistors willbe found to be satisfactory for smoothing purposes.

In the embodiment of the invention shown in the drawing, the negativeterminal of the 500 volt supply is connected to ground while thenegative terminal of the 300 volt supply is con supply. The cathodes ofall the tubes are connected to ground.

The negative bias voltage for the 807 tubes comprises the voltage dropacross the resistor RI, while the negative bias voltage for the 6V6 tubeis obtained from a voltage divider formed by the two resistors R3 and R4connected in series between the negative terminal of the 300 volt supplyand the positive terminal of the 500 volt supply. The 6V6 tube isadapted to function as a control tube as hereinafter described and thetotal negative bias voltage applied to this tube is the resultant of thevoltage across the resistors RI, R2 and R3; that produced across theresistor R3 being opposite in direction to the voltages across theresistors RI and R2.

Referring to the drawing, it is seen that there are two direct voltagecircuits having resistors R1 and R2 in common. The first of thesecircuits can be traced from the positive terminal of the 300-volt powersupply through the internal resistance of the 6V6 tube in parallel withthe internal screen-to-cathode resistances of the 807 tubes, through R1,through R2, and back to the negative terminal of the 300-volt powersupply. The point between resistor R1 and the internal resistance of thetube is at ground potential. The second of these circuits can be tracedfrom the positive terminal of the 500- volt power supply, through R4,through R3, through R2, through R1, and back to the negative terminal ofthe 500-volt power supply, which is at ground potential. Hence thecurrents flowing through R1 and R2 can be seen to oppose each other andthe resultant voltages developed across these resistors depends on thedifference in the current flowing. In the preferred embodiment of theinvention resistors R1 and R2 are traversed by the full load current ofthe 300-volt supply, and by only a current of low value derived from the500-volt power supply. In this fashion, a negative voltage is derivedacross resistor R1 and is applied to the grids of" the 807 tubes asbias. Similarly, a

negative voltage is derived across resistors R1 and R2 and; a positivevoltage, which; is some: what less than the voltage derived across re-,sistors R1 and R2, is derived across resistor R3, the resultant voltagebeing negative and being applied to the grid of the 6V6 tube When asignal is applied to the amplifier the plate current of each of the89;?- tubes,"increases thus causing a decrease in thevoltage-of the 50.volt supply which will cause a decrease in that portion of the biasvoltage applied to the-6N6 tube derived from the resistor R3. As thisportion of the bias voltage is positive, the resultant negative biasvoltage applied-to the 6V6 tube, will increase thereby causing adecrease/in theplate current of this tube. This. current flows from thecathode of the tube to the negative ter. minal of the 300 volt supply,via resistors RI and R2, and therefore the bias voltage applied;to the.

grids of the 807 tubes is decreased. At the same time, thevoltageapplied to the screen electrodes.

in the screengrid voltage of the. 807 tubes thus tending to maintaintheoptim'um power output of the tubes.

Thus a decrease in the 50.0,volt supply, due to increased plate currentwith v signal, is I compensated by a decrease in the negative grid. biasvoltage and an increase in the screen. grid voltage.

of the 807 tubes.

The. degree of compensation can bevaried if so desired by varying thevalue of theresistorB3.

In the operationof theamplifier, the flow. of, grid current in the gridcircuits of the. 807 tubes causesan increase in the, voltage. dropacross the resistor .RI, and an increasein. thernegative bias. voltageapplied to the.,80'l tubes. Theincreas ed. voltagedrop across theresistor RI also increases the negative bias voltage applied to the .6V6..tube

thus reducing the plate current flowin through;

this .tu. be. As the. plate current of .the; 6V6 .tube. fiows throughtheresistor RI -the,r,educed plate.

current of the (W6 tends to maintain. the voltage drop across theresistor RI atits original value; The negative biasvoltage applied tothe-80l tubes is thus, substantially unaffected by the flow of grid.current.

Likewise, if thescreengrid current of the 80T tubes increases,theincreased current. flows.- through the resistorsRI and-R2, therebyin-v creasing. the negative, grid. bias voltage applied to the 6V6 tubeand decreasing .thesplate current flowing, through this I tube. Sincethe plate cu rrent .of the 6V6 tube also flows through the resistors RIandRZ the reduction-in thiscurrent offsets the increased screen gridcurrent, thereby-- maintaining a substantially constantvoltagedropacrossthe resistors RI and R2; Since thescreengrid voltage for the-807tubescomprises the. dif-. ference between the 300 voltsupplyavoltageand. the voltage developedacross. the resistors Rtand- R2,it will be maintained substantiallyconstant. Consequently, thescreengrid voltage of the out-. put tubes is substantially unaffected byvariations in the screen grid current.

The decrease in th positivebomponent of the negative grid bias voltageof theGVfi tube when.

the .500; volt supply decreases. is partly '4 the decrease in voltageacross the resistors R2 andRI whentha plate current of the 6V6 tubedecreases. The decrease in the positive com ponent is also partiallyoffset by the increase in the-screen grid current of the 807 tubes whena signal is applied to the control grids of these tubes. The effect-isdegenerative but it cannot completelyoffset the increase in the negativebias voltage app1ied=to the 6V6 tube following upon the decrease in thepositive component of the bias voltage resulting from the decrease inthe voltage of the-500 vo lt supply.

In a similar manner, any changes in the voltagelo ftheBflO volt supplyfrom which the potential fonthescreen grid electrodes of the 807 tubesis takenwill produce a counter change in the bias voltageapplied to the807 tubes.

In practice, changes of voltage in the power sppplie s.-. are .mainlycaused by variations in the current consumption of the tubes followingupon changes in the level of the signal being amplified. The inventioncan substantial} overcome these variations in the. power supplyvoltages. which would otherwise limit the optimum output of the tubes.

It is. evident,that the invention may alsobe applied to modulatingsystems in which class B modulators are utilized, thus reducing thedegree of regulation required in the power supplies for the modulator,and such applications are. intended tobe Within the scope of theinvention. Having, now described our invention what we claim as new anddesire to secure by Letters Patent is:

1. An audio-frequency amplifier comprisinga driverstage having a firstelectron discharge tube.

having an anode, a control grid and a cathode, an output stage coupledto said. driver stage and havinga.se c-ond electron discharge tubehaving. an anode, acontrol grid and a cathode, a first direct.pQtfintial supply, a second direct poten- 75. electron discharge tube inseries with a portion tial supply, a resistance network connected acrosssaidsecondpotential supply, means to apply the output, potential of saidfirst. potential supply across thegridand cathode of said first electrondischarge tube, in series with a portion of said resistance network,means to apply the output potentialof said second potential supplyacross the anode and cathode of said-second electron discharge, tube,means .to connect said resistance network tothe grid of said firstelectron dischargetube, and means to connect said resistance network tothe grid of said second electron discharge tube thereby to applynegative bias voltages to the grids of said first and said secondelectron dischargetubes. at values and in series, at which achange inthe potential of either of said potential supplies will vary the biasvoltage.,applied-to the grid of said first electron discharge tube-tovary-the anode current thereof and vary. the grid bias of said secondelectron discharge tube to maintain the power output of said amplifiersubstantially constant.

2. An audio-frequency amplifier comprising a driver stage having a firstelectron discharge tubehaving'an anode, acontrol grid and a cathode anoutput stage coupled to said driver stage and havinga second-electrondischarge tube having an anode, a control gridand a cathode, a firstdirect.potentialsupply, a second direct potential supply, a resistancenetwork connected across said second potential supply, means toapplythe-output potential of said first potential supply-across.theanode and cathode of said first tential of said second potentialsupply across the anode and cathode of said second electron dischargetube, means to connect said resistance network to the grid of said firstelectron discharge tube, and means to connect said resistance network tothe grid of said second electron discharge tube to apply negative biasvoltagesto I the grids of said first and said second electron dischargetubes at values and in senses at which a change in the potential of saidpotential supplies due to flow of grid current in said second electrondischarge tube will vary the bias voltage applied to the grid of saidsecond electron discharge tube to maintain the optimum power output ofsaid amplifier substantially constant.

3. An audio-frequency amplifier comprising a driver stage having a firstelectron discharge tube having an anode, a control grid and a cathode,an output stage coupled to said driver stage and having a secondelectron discharge tube having an anode, a screen grid, a control gridand a cathode, a first direct potential supply, a second directpotential supply, a resistance network connected across said secondpotential supply, means to apply the output potential of said firstpotential supply across the anode and cathode of said first electrondischarge tube and across the screen and cathode of said second electrondischarge tube in series with a portion of said resistance network,means to apply the output potential of said second potential supplyacross the anode and cathode of said second electron discharge tube,means to connect said resistance network to the grid of said firstelectron discharge tube, and means to connect said resistance network tothe grid of said second electron discharge tube to apply negative biasvoltages to the grids of said first and said second electron dischargetubes at values and in senses at which a change in the output potentialof either of said potential supplies will vary the bias voltage appliedto the grid of said first electron discharge tube to vary the anodecurrent thereof and vary the grid bias and the screen grid voltage ofsaid second electron discharge tube to maintain the power output of saidamplifier substantially constant.

4. An audio-frequency amplifier comprising a driver stage having a firstelectron discharge tube having an anode, a control grid and a cathode,an output stage coupled to said driver stage and having a secondelectron discharge tube having an anode, a screen grid, a control gridand a cathode, a first direct potential supply, a second directpotential supply, a resistance network connected across said secondpotential supply, means to apply the output potential of said firstpotential supply across the anode and cathode of said first electrondischarge tube and across the screen and cathode of said second electrondischarge tube in series with a portion of said resistance network,means to apply the output potential of said second potential supplyacross the anode and cathode of said second electron discharge tube,means to connect said resistance network to the grid of said firstelectron discharge tube, and means to connect said resistance net-- workto the grid of said second electron discharge tube to apply negativebias voltages to the grids of said first and said second electrondischarge tubes at values and in senses at which a change in thepotential from said potential supplies due to changes in screen gridcurrent of said second electron discharge tube will vary the biasvoltage applied to the grid of said first electron discharge tube tovary the anode current thereof and vary the grid bias and the screengrid-voltage of said second electron discharge tube to maintain theoptimum power output of said amplifier substantially constant.

5. An audio-frequency amplifier comprising a driver stage having a firstelectron discharge tube having an anode, a control grid and a cathode,-an output stage coupled to said driver stage 'a'ndhaving a secondelectron discharge tube havin an anode, a screen grid, a control gridand a cathode, a'first direct'potential supply, a second directpotential supply, a resistor network comprising a first, a second, athird and a fourth resistor connected in series across said secondpotential supply said first resistor being connected to'the negativeterminal of said second potential supply, means to couple the positiveterminal of said first potential supply to the anode of said firstelectron discharge tube and to the screen grid of said second electrondischarge tube, means to connect the negative terminal of said firstpotential supply to the junction of said second and said thirdresistors, means to apply the output potential of said second potentialsupply across the anode and cathode of said second electron dischargetube, means to connect the junction of said third and said fourthresistors to the grid of said first electron discharge tube, and meansto connect the junction of said first and said second resistors to thegrid of said second electron discharge tube, and means connecting thecathodes of said electron discharge tubes to the negative terminal ofsaid second potential supply, the resistors of said network havingresistance values at which a change in the potential from said potentialsupplies will vary the bias voltage applied to the grid of said firstelectron discharge tube to vary the anode current thereof and vary thegrid bias and the screen grid voltage of said second electron dischargetube to maintain the power output of said amplifier substantiallyconstant.

6. An audio-frequency amplifier comprising a driver stage having a firstelectron discharge tube having an anode, a control grid and a cathode, apush-pull output stage having a second and a third electron dischargetube, said second and said third electron discharge tubes each having ananode, a screen grid, a control grid and a cathode, a first directpotential supply, a second direct potential supply, a resistor networkcomprising a first, a second, a third and a fourth resistor connected inseries across said second potential supply said first resistor beingconnected to the negative terminal of said second potential supply, atransformer having a primary and a center-tapped secondary winding,means connecting one end of the primary winding of said transformer tothe anode of said first elec tron discharge tube, means connecting thepositive terminal of said first potential supply to the other end of theprimary winding of said transformer and to the screen grids of saidsecond and said third electron discharge tubes, means to connect thenegative terminal of said first potential supply to the junction of saidsecond and said third resistors,. means to couple the junction of saidthird and said fourth resistors to the grid of said first electrondischarge tube, and means to connect the junction of said first and saidsecond resistors to the center tap of the secondary winding of saidtransformer, means to connect the ends of said secondary winding to thegrids of said second and said third electron

